1. Field of the Invention
The present invention relates generally to locomotive propulsion, and more particularly, to the control of wheel slip and wheel slide in locomotive traction wheels during acceleration and braking.
2. Description of the Prior Art
A typical rail locomotive has four or six drive axles. Each drive axle extends between a pair of traction wheels that are disposed for rotational engagement with the track rails. A motor is coupled with each drive axle through a gearing assembly in such a fashion that rotation of the motor armature rotates the drive axle in a predetermined ratio, which in turn rotates the traction wheels mounted on its ends. Accordingly, propulsion of the locomotive is achieved by exciting the motors adapted to rotate the drive axles and traction wheels.
It has long been known that the metal-metal surface contact between the traction wheels and the track rail fosters a variable frictional engagement. As a result, wheel slip has long been known to be a factor in locomotive speed control, and excessive wheel slip has been known to be a problem, particularly during times of acceleration, braking, and changing wheel-rail surface conditions. Among the deleterious effects of excessive or uncontrolled wheel slip are the reduction in acceleration and braking efficiency, reduced tractive effort or adhesion, and excessive component wear and tear leading to premature fatigue and failure of various component parts.
As a preliminary matter of clarification, the term wheel slip, as it will often be used in this specification unless otherwise indicated, refers to either positive wheel slip as incurred during times of normal motoring operation or negative wheel slip, or wheel slide, as incurred during times of braking. It will be appreciated that the corrective actions taught by the present invention can be applied to control systems to compensate for positive wheel slip, negative wheel slip, or both. During positive wheel slip power to the drive motors will be reduced, whereas during negative wheel slip braking power will be reduced.
In addition to times of acceleration and braking, wheel slip is often problematic when the locomotive is traveling at a constant velocity. For example, when track conditions suddenly change and the locomotive travels from a dry track section to a wet or oily track section, wheel slip can occur. Since track conditions can change virtually instantaneously, it is important for the wheel slip corrective system to be able to quickly detect and immediately act upon a slip condition.
Early wheel slip corrective systems approached the problem by detecting wheel slip and eliminating it entirely. It was later determined that a small amount of wheel slip was desirable. More specifically, it was discovered that increased locomotive tractive effort and, therefore, enhanced locomotive performance was achieved at slip levels as high as ten to twenty percent, depending upon the track conditions. Therefore, later wheel slip corrective systems sought to enhance locomotive performance by maintaining a small percentage of wheel slip. Indeed, a number of presently employed wheel slip corrective systems operate in this fashion, ignoring small amounts of wheel slip and taking corrective action only after a certain threshold of slip has been exceeded. Other, more sophisticated systems, operate to maintain a variable amount of wheel slip, in an attempt to maintain maximum traction throughout changing track conditions.
While these and similar corrective systems enhance locomotive performance, further improvements are desired. Improvements, for example, that will further reduce the time required for wheel slip correction, while, at the same time, further improving locomotive performance. It should be appreciated, however, that competing interests (fast wheel slip correction and maintaining high locomotive performance) are present in a wheel slip correction sequence. On one hand, since excessive wheel slip acts to degrade locomotive performance, locomotive performance is enhanced by quickly eliminating excessive wheel slip. On the other hand, however, inertial effects degrade locomotive performance when sharp power reductions are applied to the traction motors.
To better illustrate this latter point, suppose a locomotive is accelerating from rest and the drive motors are operating at a particular power level when excessive wheel slip is detected. Wheel slip can be quickly reduced or eliminated by abruptly reducing power to the drive motors. However, such a sharp power reduction will result in an undue delay in the locomotive acceleration (and rough train handling), and thus degrade locomotive performance. Furthermore, power must be reapplied in a controlled manner in order to avoid applying excessive strain on various coupler components.
Accordingly, it is desired to maximize locomotive performance by rapidly reducing excessive wheel slip, while minimizing the power reductions applied to the motors. This is more commonly referred to as minimizing the power "deration" (i.e., amount and duration of corrective action).